Page 310 - Trenchless Technology Piping Installation and Inspection

274 Cha pte r S i x

10 hours, strength would be appropriate, in combination with antici-
pated values of f and f during this period. For the postinstallation
o R
phase, a 1000-hour collapse strength is conveniently assumed as which
time the soil has locally redistributed itself around the pipe to provide
some lateral constraint against collapse, in combination with the rela-
tively tight clearance between the replacement pipe and the expanded
borehole walls, characteristic of a pipe-bursting operation. For this
postinstallation phase, the tension reduction factor, f , is equal to 1.0.
R
Based on the ovality reduction factor, dependency trend provided
in ASTM F1962, and consistent with the present simplified approach,
it is reasonable to assume a maximum overall value of f of approxi-
o
mately 0.5 to account for ring deformation due to initial ovality plus
that induced by postinstallation loadings.
The greater short-term (or 10-hour) collapse strength relative to
that at 1000 hours would tend to be offset somewhat by the tension
reduction factor, f , during installation, as well as the degradation at
R
possible elevated temperature as the HDPE pipe sits on the surface
prior to installation. However, the latter effects would not be experi-
enced simultaneously, as the pipe is placed at its maximum depth
and distance. Therefore, the 1000-hour (postinstallation) collapse
characteristics, as adjusted for the above soil density and ovality con-
siderations, are conveniently employed to evaluate the vulnerability
to collapse. Based upon the above discussion, the following planning
guide is determined:
H (ft) = 1000 hour water head × f × f /γ
all o R
= 1000 hour water head × (0.5) × (1.0)/(2.0)

or,

H (ft) = 1000 hour water head/4.0 (6.7)
all
where H is the allowable (maximum) pipe depth.
all
An additional degree of margin (safety factor) may exist due to
soil arching effects, reducing the effective load on the pipe), as well as
the aforementioned degree of lateral constraint of the pipe provided
within the cavity.
As an example, consider the feasibility of installing a 4-in. DR 17
pipe at a depth of 12 ft. Applying Eq. (6.7) results in

H (ft) = 1000 hour water head/4.0
all
= 62 ft/4.0
= 15.5 ft

Thus, the present planning guide indicates that the proposed 12-ft
depth is reasonable.